In previous square bottles, the width of the corner portions was approximately fixed in the vertical direction. Those corner walls functioned as pillars to hold the bottle rigid. Even when the interior of the bottle was in a somewhat decompressed state, those corner walls suppressed the deformations.
When the width of corner walls is expanded from the top-end portion and the bottom-end portion toward approximately the mid-height position, the function of preserving rigidity with respect, especially, to a bottle flattening deformation of such corner walls declines; the corner walls themselves deform in even a relatively low decompression state, and there is a tendency for visually prominent deformations such as flattening of the bottle shape to occur.
For example, Japanese Laid Open Patent 2590084 discloses a square bottle manufactured from synthetic resin (referred to below as simply a “square bottle”), being an approximately square tube wherein a trunk portion comprises four side walls and four corner walls positioned between the side walls, in which decompression absorption panels are formed in the side walls. These panels function to uniformly absorb decompression, so that deformations caused by negative pressure when the interior is decompressed are not visually prominent.